The use of implanted medical devices to provide electrical stimulation therapy to individuals for various medical conditions has become more widespread in recent times. This has occurred as the advantages and benefits such devices provide become more widely appreciated and accepted throughout the population.
Electrical stimulation can be used for therapeutic effect at various locations within the body. For example, functional electrical stimulation systems may be used to deliver electrical pulses to certain neurons of a recipient to cause a controlled movement of a limb of such a recipient. Also, electrical nerve stimulators are used in applications such as cochlear implants for restoring hearing to people with sensorineural hearing loss.
An implantable hearing prosthesis can provide the benefit of hearing to individuals suffering from severe to profound sensorineural hearing loss. Sensorineural hearing loss is due to the absence or destruction of the hair cells in the cochlea which transduce acoustic signals into nerve impulses. An implantable hearing prosthesis simulates the cochlear hair cells by delivering electrical stimulation to the auditory nerve fibres. This causes the brain to perceive a hearing sensation.
Electrical stimulators pass electric current pulses through nerve tissue causing the nerves to depolarise and fire. The timing and magnitude of the current is usually controllable so that the timing and number of nerves fired is controlled. Each nerve has an energy threshold at which it fires and a time, known as the refractory period, before it can be fired again after stimulation. A problem exists, therefore, when the signals to the electrical stimulator require a nerve to be stimulated repeatedly prior to that nerve being reset to fire again.
It is generally desirable that electrical stimulation systems such as the noted implantable hearing prosthesis consume minimal power. Lower power consumption leads to smaller components and longer battery life.
Pharmaceutical agents can modify the response of nerves to electrical stimulation and induce neural firing in their own right. They are known to be delivered into the area to be treated by a pump mechanism through a tube. The position where the agent is injected is controlled by the placement of the end of the tube and the concentration of agent varies considerably at different parts of the structure.
It is an object of the present invention to provide an improved method and apparatus for the delivery of pharmaceutical agents using implantable devices.